DESCRIPTION: Using a psychophysical method that reliably defines the limits of spatial, tactile acuity, the PI has shown that patients with PD have greatly elevated thresholds for spatial discrimination. The objective of the studies proposed here is to investigate the neural mechanisms that underlie this decline in sensory performance. 28 patients with PD will be studied over a period of 5 years. Each will undergo a longitudinal study involving psychophysical testing and PET imaging while performing the psychophysical tasks. The psychophysical experiments measure the patients capacity for spatial discrimination 6 months before surgical pallidotomy, just before pallidotomy, and 4 months after pallidotomy. In the sessions just before surgery and at the 4-month follow-up, the patients are assessed on two successive days while they on and off their medication. Two groups of normal subjects will undergo the same psychophysical testing and PET scanning. The first will consist of 28 subjects who are matched for age, gender, and handedness (and also for socioeconomic status and education to the degree possible). The second group will consist of 20 younger subjects, 20-30 years old. The first specific aim is to investigate the patients capacity for tactile spatial discrimination in greater detail. The testing sessions at 6 months before and just before pallidotomy will characterize the longitudinal stability of patients sensory capacity. The testing sessions on successive days when patients are on and off medication, will test the effects of L-dopa on sensory performance. The sessions 4 months after surgery will provide an index of the effects of pallidotomy. The second specific aim is to investigate the effects of aging on rCBF. All of the normal control subjects will undergo PET scanning while being testing for tactile spatial discrimination. These studies will identify any changes in rCBF that are correlated with the normal, age-related decline in tactile spatial acuity and they will provide age-matched controls for comparison with the PD patients. They will also determine degree to which rCBF changes are related to a specific hemisphere, handedness, and the side being stimulated. The third specific aim is to investigate the neural correlates of impaired tactile spatial discrimination in PD patients and how rCBF is affected by medication and by pallidotomy. The first will be done by comparing rCBF patterns in PD patients with age-matched control subjects. The second will be done by longitudinal comparisons within subjects; that is comparisons of rCBF patterns on and off medication and before and after pallidotomy.